The present invention is directed to an improvement in a rotary vacuum precoat filter. More particularly, the invention is directed to an improvement in a rotary vacuum precoat filter for use in filtering hot, unsaturated, saturated or supersaturated slurry. In a more particular sense, the improved rotary vacuum precoat filter herein has utility for use in filtering a phosphoric acid slurry formed during the manufacture of phosphoric acid.
Phosphate rock is mined principally for conversion to phosphoric acid and thereafter to high phosphate fertilizer products. Mined phosphate rock usually after beneficiation, is solubilized with phosphoric acid and reacted with sulfuric acid to produce phosphoric acid solution and insoluble calcium sulfate. Such a process is generally referred to in the industry as the wet process of phosphoric acid production. The resultant phosphoric acid solution is further processed to produce fertilizer products.
The oldest and most economical method for making crude phosphoric acid is to dissolve beneficiated phosphate rock in phosphoric acid which usually contains some sulfate ions and then to complete precipitation of calcium by adding sulfuric acid, thereby precipitating calcium sulfate and releasing phosphoric acid (wet process).
In the evolution of phosphoric acid production, the processes for making phosphoric acid have been named in relation to the by-product produced during the process. For example, the phosphoric acid production processes are known as the gypsum process (more commonly the dihydrate process), the hemihydrate process and the anhydrite process. All three processes are named with regard to the by-product calcium sulfate produced. The gypsum or dihydrate process is conducted at a temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration of about 30% in the liquid portion of the slurry. Increasing the temperature from about 90.degree. to 120.degree. C. and the P.sub.2 O.sub.5 concentration from about 30% to about 45% in the liquid phase produces calcium sulfate hemihydrate. If, however, one chooses to run the phosphoric acid wet process at 130.degree. C. and a P.sub.2 O.sub.5 concentration greater than 30%, then one obtains calcium sulfate anhydrite as the by-product. For further detail on the preferred conditions for each type process see U.S. Pat. No. 4,196,172 of Ore et al issued Apr. 1, 1980, incorporated herein by this reference. The processes are more fully described in "Phosphoric Acid," Parts I and II, edited by A. V. Slack, Marcel Dekker, Inc., New York, N.Y., 1968.
Generally, in a hemihydrate process, phosphate rock and sulfuric acid are reacted with a slurry comprising phosphoric acid, sulfuric acid, monocalcium phosphate and calcium sulfate hemihydrate. The phosphoric acid in the slurry can be a recycled process phosphoric acid or a recycling of the slurry. The temperatures and P.sub.2 O.sub.5 concentrations are such that the main product from the reaction of phosphate rock and the sulfuric acid is calcium sulfate hemihydrate and phosphoric acid. Little, if any, calcium sulfate dihydrate is observed in the reaction. The slurry so produced is then sent to a recovery section where the solids are separated from the liquid. The bulk of the solids to be separated comprises the by-product calcium sulfate and for this process calcium sulfate hemihydrate. Repulping of the calcium sulfate hemihydrate in water and sulfuric acid and recrystallization to gypsum can be useful in increasing phosphoric acid yield.
Separation of the calcium sulfate hemihydrate, or gypsum from repulping, and other solids from the slurry is usually accomplished by means of a filter or less preferably by a centrifuge. Many filters are available for separating the solids from the slurry. The rate of filtration or the filterability of the slurry is dependent, among other things, upon the size, shape (or crystal habit) and size distribution of the calcium sulfate hemihydrate crystals produced during the reaction.
The crude phosphoric acid recovered after filtration of the calcium sulfate by-product assays from 22 to 45 percent P.sub.2 O.sub.5 and is subsequently concentrated by evaporation to a P.sub.2 O.sub.5 assay of about 50%. The concentrated crude phosphoric acid can be utilized in the production of fertilizers or can be treated to produce high grade phosphoric acid or other phosphates.
The crude phosphoric acid includes substantial proportions of impurities, such as fluorine; calcium, determined as CaO; sodium, expressed analytically as Na.sub.2 O; SiO.sub.2 ; SO.sub.4 ; iron, determined as Fe.sub.2 O.sub.3 ; aluminum, determined as Al.sub.2 O.sub.3 ; and magnesium determined as MgO. The relative proportions of impurities contained in the crude phosphoric acid are largely determined by the type of wet processing utilized to produce the crude phosphoric acid and the nature of the phosphate-bearing rock from which the crude phosphoric acid is produced. Sufficient proportions of the impurities remain in the acid even after aging and settling and after a substantial period of time, separate from the acid in the form of a sludge. It is commercially impractical to hold the crude phosphoric acid sufficiently long enough to allow the formation and separation of the sludge and consequently the sludge is normally encountered at the user's location, often resulting in rejection of the acid.
It has been found, as disclosed in U.S. Pat. No. 4,235,854, filed of Smith et al, that by preheating the acid, followed by vacuum filtration through a filter medium including a diatomite cake, sludge forming impurities can be removed in sufficient quantity to substantially eliminate the subsequent formation of sludge during shipping and handling of the phosphoric acid product. The above U.S. Pat. No. 4,235,854 is assigned to the same assignee (or parent corporation of the present assignee) and the entire disclosure thereof is incorporated herein by this reference.
Sludge forming impurities can be removed from phosphoric acid by the process disclosed in U.S. Pat. No. 4,136,199, the entire disclosure of which is incorporated herein by this reference.
Preferably, the filtration of the crude phosphoric acid is conducted using a rotary vacuum precoat filter. Rotary vacuum precoat filters are well-known in the art. Such rotary vacuum precoat filters which typically comprise rotating a cylindrical filter drum to continuously submerge a segment or portion of the peripheral screen filtering medium consisting essentially of a permeable bed or cake of filter aid, into the suspension or slurry of liquids and solids to be filtered, the pressure differential inducing passage of the liquid filtrate component of the suspension or slurry through the filtering medium, thereby separating the liquid from the solids, which are retained on the surface or entrapped within the interstices of the present cake adjacent to the surface. Collection and retention of the solids fill and can block the pores or interstices, inhibiting further passage and separation of filtrate which necessitates the removal of the surface portion of the precoat cake containing the retained and entrapped solids to permit further filtration.
In many vacuum precoat filters, the removal of retained solids is typically effected by means of a continuously advancing doctor knife or blade which penetrates into the precoat cake to a depth approximately that reached by the entrapped solids. Thus, uninterrupted filtration through rotation of the cylindrical filter drum is permitted by the continuous cutting away and removing of the precoat cake and entrapped solids and in turn exposing a substantially uncontaminated new surface of precoat cake for further filtration.
Once the progressively advancing knife reduces the precoat cake to the minimum thickness necessary for satisfactory filtration, the filtering cycle is halted while a new or additional precoat is applied.
With the use of rotary vacuum precoat filters, as well as other types of precoat filters, some P.sub.2 O.sub.5 values can be lost in the precoating of filter aid. That is, the precoat cake can entrap and retain P.sub.2 O.sub.5 values in the interstices of the cake. It would be desirable to develop apparatus and a method for recovering such P.sub.2 O.sub.5 values.